DCF77 interface

Posted on Feb 6, 2014

The circuit interfaces a commercial DCF77 receiver (available for about $15) with a PC using the RS232 serial port. By rewriting the program and/or modifying the circuit it`s possible to use also different systems or different I/O ports. The power is driven from three lines of the RS232 port (that is full protected against short circuits and there

DCF77 interface
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fore there is no risk of damaging it). The DTR and RTS lines are set high (+12V on the port) and the TXD is kept low ( 12V) for the power supply of the receiver. The RTS line gives positive power to the circuit and the DTR powers only the monitor LED that is kept separated from the rest of the circuit because of its high current peaks. The BF244 JFET, the 1. 5k © resistor and the second LED compose a voltage reference of about 1. 8V needed for powering the commercial receiver. Only a very low current flows through this LED won`t be particularly bright. The TL081 sends the DCF77 signal (RS232 compatible) on the RXD pin. This pin must always be high ( 12V since this pin is inverted on the RS232) and go low one per second (when a bit is received). The DCF77 standard imposes a pulse time of 200ms for high bit and a of 100ms for low bit, but, since receivers usually use about 40ms for a safe reception these times are reduced to 160 and 60ms. When receiving this pulses with the PC UART 8250 (or better) at the speed of about 40 bit per second N81 a 160ms pulse will produce a $80 or a $C0 byte and a 60ms a $FE or $FC.

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